WASHINGTON - US Secretary of State Mike Pompeo was on his way to North Korea on Tuesday to prepare for an unprecedented summit with North Korean leader Kim Jong Un as President Donald Trump signaled the possibility that three Americans detained there could soon be released.Breaking the news of Pompeo's second visit to North Korea in less than six weeks, Trump said America's chief diplomat was expected to arrive “very shortly” and that the two countries had agreed on a date and location for the unprecedented summit, though he stopped short of providing details.While Trump said it would be a "great thing" if the detainees were freed, Pompeo, speaking to reporters en route to Pyongyang, said he had not received such a commitment but hoped North Korea would "do the right thing." His visit, he said, was intended to finalize a summit agenda that could enable a "historic, big change" in relations between long-time foes.The detainees' release could signal an effort by Kim to set a more positive tone for the summit, which is being planned for late May or early June, following his recent pledge to suspend missile tests and shut Pyongyang’s nuclear bomb test site.While Kim would be giving up the last of his American prisoners, whom North Korea has often used as bargaining chips with the United States, a release could also be aimed at pressuring Trump to make concessions of his own in his bid to get Pyongyang to abandon its nuclear arsenal, something it has not signaled a willingness to do."Plans are being made, relationships are building," Trump said of the planned summit during remarks otherwise focused on his decision to pull the United States out of the 2015 Iran nuclear deal."Hopefully, a deal will happen. And with the help of China, South Korea and Japan, a future of great prosperity and security can be achieved for everyone," Trump added.Pompeo made a secret visit to North Korea over the Easter weekend, becoming the first U.S. official known to have met Kim, to lay the groundwork for the planned summit. The meeting occurred before Pompeo's Senate confirmation as secretary of state.Trump suggested that dropping out of the Iran nuclear accord, which he has frequently denounced as a bad deal, would send a "critical message" not just to Tehran but also to Pyongyang."The United States no longer makes empty threats. When I make promises, I keep them," Trump said.But critics of Trump's decision to leave the Iran deal say it could undermine his credibility in North Korea's eyes, fueling doubts whether he would abide by any nuclear agreement.FATE OF THREE DETAINEESPompeo’s latest trip raised the prospects that the three Korean-American detainees - Kim Hak-song, Kim Sang-duk and Kim Dong-chul - could be turned over to him.Asked whether that could happen, Trump told reporters: “We’ll soon be finding out. It would be a great thing if they are.”Pompeo, whom aides say has played a key role in negotiations with North Korea on the issue, said, "We have been asking for the release of these detainees for ... 17 months," according to a transcript provided by the State Department of his remarks to reporters aboard his plane."We'll talk about it again today," he said. "I think it'd be a great gesture if they would choose to do so."Pompeo said he was hoping to nail down a framework for the summit, the first-ever meeting of sitting U.S. and North Korean leaders. Trump has said the meeting could take place at either the heavily fortified demilitarized zone between North and South Korea or in Singapore.Pompeo, when he was still the CIA director, met Kim on this last trip but said he did not know whom he would meet this time. If he does see Kim again, he would be only the second secretary of state to sit down with a North Korean leader. The last was Secretary of State Madeleine Albright, who went on an unsuccessful trip in 2000 to arrange a meeting between President Bill Clinton and Kim's father, Kim Jong Il.Pompeo said he hoped to set out conditions to allow Trump to achieve the goal of "complete, verifiable, irreversible denuclearization" of North Korea, but insisted that sanctions would not be lifted before that."We are not going to head back down the path that we headed down before," he said. "We're not going to relieve sanctions until such time as we achieve our objectives."A senior State Department official said Washington would be looking for "bold steps" by North Korea rather than incremental agreements on nuclear disarmament that Pyongyang has violated in the past.Pompeo's latest visit followed talks between Kim and South Korean President Moon Jae-in on April 27 at the demilitarized zone, the first summit for the two Koreas in over a decade.The North-South summit produced a declaration of goodwill but was short on specific commitments and failed to clear up the question of whether Pyongyang is really willing to give up nuclear missiles that now threaten the United States.US officials have been pressing Kim to free the three remaining American detainees as a show of sincerity before the summit. Trump and Kim have exchanged insults and threats over the past year but tensions have eased in recent months.Until now, the only American released by North Korea during Trump’s presidency was Otto Warmbier, a 22-year-old university student who returned to the United States in a coma last summer after 17 months of captivity and died days later.Warmbier's death escalated US-North Korea tensions, already running high at the time over Pyongyang's stepped-up missile tests.The three still being held are Korean-American missionary Kim Dong-chul; Kim Sang-duk, also known as Tony Kim, who spent a month teaching at the foreign-funded Pyongyang University of Science and Technology (PUST) before he was arrested in 2017; and Kim Hak-song, who also taught at PUST.